The single-phase BLDCM requires a suitable current commutation signal synchronized with the rotor position for proper operation. In most applications, a Hall-effect position sensor is used to detect the rotor position and control the motor. However, the Hall-effect sensor itself increases the size of the motor system and the manufacturing costs. Besides, the Hall-effect sensor decreases the capability of the system against the environment variation, e.g., the temperature variation. Many Hall-less BLDCM drives that detect the rotor position have been introduced. FIG. 1 shows the configuration of a conventional single-phase BLDCM having a stator and a rotor.
In order to avoid the use of the Hall-effect sensor, a method employing a sensing coil to determine the rotor position is proposed in U.S. Pat. No. 5,598,071, and the configuration of the disclosed single-phase BLDCM having a stator and a rotor is shown in FIG. 2. Referring to FIG. 2, the BLDCM includes a magnet 3, a coil 4, a starter winding 5, an air gap 8, a rotor 9, an opening 11, and a stator 12 having four poles (1 and 2). In which, the arrows such as 6 and 7 are employed to indicate that the magnet 3 is radically magnetized, the magnetic field distribution is asymmetrical about axis 9, the coil 4 is wrapped around the poles 2 of stator 12, and the starter winding 5 is fitted around the poles 1 of stator 12. Besides, the starter winding 5 has sufficiently large number of turns to get a high back electromotive force (BEMF) constant and be used to sense the BEMF signal. Furthermore, a controller can also control the BLDCM successfully with the BEMF signal
However, the additional poles 1 are introduced to the stator 12, which makes the motor configuration more complicated and the space for placing the coil 4 smaller.
FIG. 3 shows the relationship between the BEMF and the armature current of stator winding of the conventional BLDCM having a sensor. The motor is controlled through two steps S1 and S2. In each step, the controller gets the rotor position from the sensor, e.g., a Hall sensor, and analyzes the BEMF signal, then makes it possible that the armature current just has the relationship with BEMF as shown in FIG. 3 by controlling the four switches of the H-bridge of the stator.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicants finally conceived the method and circuit for controlling a sensorless single-phase BLDCM.